Hi aggie
I'm, the Car Hire expert here at travelsupermarket.com so hopefully be able to help you clear up any queries on this.
Jim is spot on advising you to contact the company to explain the conditions that you aren't sure about and if you are still not clear on any of them give them a call back as they are in the best position to help you understand them.
Generally what you will find is that Collision Damage Waiver (or Loss Damage Waiver as its more commonly known in the States) is usually included in the price. What this gives you is insurance cover to drive the car much like your own insurance for your car you drive over here. This means that you will be insured but with an excess to pay in the case of any accident/damage caused by the driver.
This is where the extra insurance cover comes in to play. Companies will try and get you to sign up for a reduced excess at a cost when you are picking the car up (you can also mostly buy this at time of booking on the web site also). Alternatively there are companies out there such as insurance4carhire who offer this excess reduction at a cost also.
My biggest bit of advice that I will give to anyone and everyone is to read the terms and conditions fully before confirming any booking (this includes any excess reduction insurance you might take out) and agin as Jim advised call them if you are unsure. This gives you a clear idea of how much you are paying and should'nt then give you any surprise charges later on but always check your credit card to make sure no unauthorised charges have been taken out
I hope the above (apologies for the length) is of help to you but if yo have any other questions please don't hesitate to contact me on peter.edwards @ travelsupermarket.com (remove spaces around the @)
Pete